1. Field of the Invention
The present invention relates to a cantilever for use in an atomic force microscope. The present invention also relates to a method of manufacturing such a cantilever, an atomic force microscope and a sample surface adhesion evaluating method each using the cantilever.
2. Description of the Related Art
Hitherto, adhesion of a resist film which is formed on silicon substrates, for example, has been evaluated by scratching the formed resist film with a testing machine and visually observing a tear or scratch produced in the resist film. Alternatively, the adhesion has been evaluated by producing a cut through a resist film with a knife, sticking an adhesive tape onto the cut film surface and then peeling the tape off therefrom, and visually observing the degree by which the resist film is peeled off with the adhesive tape.
In other words, since adhesion between the sample surface and a film formed thereon was evaluated by visual inspection in the past, it was impossible to carry out quantitative evaluation. Therefore, a demand for any method which is capable of evaluating the adhesion in a quantitative manner has existed.
Meanwhile, there is known an atomic force microscope of the type utilizing an atomic force that is generated, when a probe provided on a cantilever comes closer to the sample surface, due to the van der Waals force between the probe and the sample. More specifically, the cantilever is relatively scanned over the sample surface with an accuracy on the atomic order while the sample position is controlled so that a warping of the cantilever caused by the atomic force is kept constant. An image showing unevenness of the sample surface is depicted from resultant control amounts.
A conventional general atomic force microscope includes a cantilever as shown in FIG. 17. The cantilever comprises a cantilever body 1 which is fixed to one end of a glass base plate 3 having dimensions of about 2 mm.times.5 mm and has a V-shape in its plan view, and a probe 2 at the distal end of the cantilever body 1. By way of example, the cantilever body 1 and the probe 2 are each made of silicon nitride, and the probe 1 in the form of a pyramid with sides of its bottom surface being 5 .mu.m is formed at the distal end of the cantilever body 1 which is 100 .mu.m long and 0.7 .mu.m thick. The cantilever has a spring constant of 0.58 N/m and a resonance frequency of 77 kHz.
A method of manufacturing such a cantilever is shown fin FIGS. 18a-18i. First, a resist 5 is applied to the surface of a (100) silicon substrate 4 as shown in FIG. 18a, and a portion of the resist 5 is removed in a rectangular pattern as shown in FIG. 18b. Then, as shown in FIG. 18c, the silicon substrate 4 is subjected to the wet etching process with the resist 5 serving as a mask, so that the silicon substrate 4 is selectively etched in the (111) direction to form an etch pit 4a. After that, the resist 5 is removed as shown in FIG. 18d, and a silicon nitride film 1a is formed over the entire surface of the silicon substrate 4 as shown in FIG. 18e.
Subsequently, as shown in FIG. 18f, a glass plate 3a is bonded onto the silicon nitride film 1a. The surface of the glass plate 3a facing the silicon nitride film 1a is divided into two areas by a previously prepared saw cut 3b. A chromium coating 3c serving to release the glass surface from the silicon nitride film 1a is formed on the surface area of the glass plate 3a which lies above the etch pit 4a in the silicon substrate 4. Then, as shown in FIG. 18g, the glass plate 3a is saw-cut through the remaining portion behind the saw cut 3b for completely dividing the glass plate 3a into portions 3d and 3e. Thereafter, as shown in FIG. 18h, the glass plate portion 3e lying above the etch pit 4a is removed. Finally, by removing the silicon substrate 4, a cantilever as shown in FIG. 18i is obtained.
The operation of the conventional atomic force microscope will be described with reference with FIG. 19. In the atomic force microscope, a repulsion acts between the probe 2 at the distal end of the cantilever and a sample 8 due to the van der Waals force between the atoms in the probe 2 and those in the sample 8, causing the cantilever to warp. A laser light 6a emitted from a laser oscillator 6 is focused through a lens 10 onto an upper surface of the cantilever body 1 at the distal end, and the reflected light therefrom enters a photodetector 7. When the cantilever warps, the position where the reflected light enters the photodetector 7 is varied and, therefore, a minute "warp" of the cantilever can be detected from the incident position of the reflected light. A piezoelectric device 9 is operated to scan in each of the X and Y directions while being subjected to feedback control in the Z direction so that the incident position of the reflected light onto the photodetector 7 is kept fixed. An image showing surface unevenness of the sample 8 can be output on a display by using the voltages applied to the piezoelectric device 9 during the scan in relation to the X, Y and Z directions.
In the atomic force microscope thus constructed, the cantilever body 1 is made of a high sensitivity material, i.e., silicon nitride, so that the cantilever body 1 is displaced by a very minute force on the order of 10.sup.-7 to 10.sup.-9. Taking into account convenience in manufacture of the microscope, too, the probe 2 is also made of the same material, i.e., silicon nitride.
As described above, adhesion of the sample surface has conventionally been evaluated by damaging a film formed on the sample surface by scratching or intentionally peeling off the film, and then visually inspecting a degree of the damage. This has raised problems that the adhesion cannot be evaluated quantitatively and evaluation of the adhesion entails damage of the sample surface on which the film has already been formed.
The present invention has been made to solve the above problems and, since an atomic force acting between a probe and the sample surface can be measured in atomic force microscopes, it is intended to measure such an atomic force and to quantitatively evaluate adhesion between the sample surface and a substance formed thereon based on a finding that adhesion between the sample surface and a substance formed thereon is related with the atomic force acting between the substance and the sample surface.
In an attempt to detect an atomic force acting between a substance M1 forming a probe and a substance M2 forming the sample surface for evaluating the adhesion produced when the substances M1 and M2 are made adhere to each other by the atomic force, only the atomic force between silicon nitride and the material M2 could be measured in the past because the substance M1 forming the probe was limited to silicon nitride from the standpoint of sensitivity.
Meanwhile, when manufacturing semiconductor integrated circuit devices, it is necessary to adhesively form, in addition to a nitride film, other various films such as an oxide film, polysilicon film and aluminum film on the surfaces of semiconductor substrates, and to form some of those films as laminated layers. Furthermore, resist patterns are often formed on those various films in photolithography steps for the purpose of patterning the films. Accordingly, there has also been a need for evaluating not only the adhesion between silicon nitride and any other substance, but also the adhesion produced when two substances other than silicon nitride are formed in mutually adhering relation.
However, since the conventional atomic force microscope described above could measure only the adhesion between silicon nitride and the other substance M2, as previously discussed, it was impossible to evaluate the adhesion produced when two substances other than silicon nitride are formed in mutually adhering relation, by using the conventional atomic force microscope.